This invention relates to sensors such as an immuno sensor, gas sensor or ion sensor using surface plasmon resonance phenomenon, and to a device using these sensors.
The surface plasmon is a propagating wave of collectively oscillating free electrons at an interface between a thin metallic film and a dielectric material. As the propagation of the wave is sensitively influenced by the dielectric constant at the interface, it forms a basis for a detection principle in immuno sensors and gas sensors. A typical structure of such a sensor is shown in FIG. 2A. A thin metallic film 21 made of free electron metal such as gold or silver with thickness of approximately 50 nm is formed on the surface of a prism with a high refractive index, and which in addition to the thin metallic film 21 has a molecule recognition layer 29. To generate the surface plasmon at the surface of the thin metallic film 21, the film 21 must be irradiated from the prism side by p polarized collimated monochromatic light 22 from a light source 23 at a particular angle called resonance angle. Generation of the surface plasmon is monitored by detecting specularly reflected light 25 with a detector 26. In other words, as shown in FIG. 2B, when the incident angle matches the resonance angle 27 at which the surface plasmon is excited, the intensity 28 of the reflected light becomes extremely small because a large portion of the energy in the incident light becomes transferred to the surface plasmon. The resonance angle depends on the dielectric constant at the surface; when change in the dielectric constant is induced at the surface of the thin metallic film 21, the intensity 31 of the reflected light now decreases at a new resonance angle 30. Because the resonance angle depends only on the dielectric constant within the region of several hundreds of nm from the surface, adsorption of a small amount of sample is enough to shift the resonance angle. Now if a device is constructed with a molecule recognition layer 29 capable of recognizing and binding a specific molecule, and if a sample fluid is allowed to flow over the surface, the dielectric constant will vary if a specific molecule in the sample fluid is bound. Hence, by observing the light reflected around the resonance angle, it can immediately be known if the specific molecule has been captured by the molecule recognition layer 29.
In order to make use of the surface plasmon phenomenon associated with a thin metallic film, apart from the above collimated monochromatic light, a method (1) is known where the film is irradiated by divergent monochromatic light and the reflected light is measured by a light sensor array, and a method (2) where the film is irradiated by collimated white light, and the reflected light is measured by a spectrometer. In both methods it is the dependence of the resonance angle for light of a specific wavelength on the dielectric constant of the interface which is used.